Painting has long been the process of choice for applying coatings to surfaces especially those having complex curvature. Painting is generally a controllable, reliable, and versatile process. The paint can include additives to give the surface desired physical properties, such as gloss, color, reflectivity, or combinations thereof. The painting process is well understood and produces quality coatings having uniform properties even when the surface includes complex curvature. Unfortunately, painting is falling under closer environmental scrutiny because it uses volatile solvents to carry the pigments or because of the pigments themselves. Therefore, there is a need to replace the painting process with a process that has less environmental impact. Furthermore, while painting is well defined, well understood, and common, it remains an “art” where masters produce better products than novices or apprentices without necessarily being able to account for why or to teach others how.
Painted surfaces sometimes lack the durability that quality-conscious customers demand. The surface must be treated and cleaned prior to applying the paint. The environment surrounding the part must be controlled to capture volatile organic compounds (VOCS) during the coating application, often requiring a spray booth. Painted coatings are also vulnerable to damage like cracks or scratches. Isolated damage may require the repair of a large area, such as forcing the repainting of an entire panel.
Spraying inherently wastes paint and is unpredictable because of the “art” involved with the application. Improper application cannot be detected until the spraying is complete, then rework to correct a defect usually affects a large area even for a small glitch. Spraying large objects like aircraft requires special equipment and special capital facilities, for example, paint spray booths, in which the environment and flow conditions are controlled.
U.S. Pat. No. 4,986,496 (Marentic et al.) describes a drag reduction article in the form of a conformable sheet material (a decal) with surface texturing for application to aircraft flow control surfaces to reduce aircraft drag. The material fits on curved surfaces without cracks, bubbles, or wrinkles because of the paint-like properties of the basic carrier film. Marentic et al.'s decals are manufactured flat and are stretched to the intended simple curvature. Stretching can be problematic over time if the stretched material shrinks to expose a gap between adjacent decals where weather can attack the decal-surface interface. Stretching generally limits Marentic appliqués to surfaces of slowly changing curvature.
Appliqués (that is, decals) are also described in U.S. Pat. No. 5,660,667 (Davis). The appliqués having complex curvature form complete, bubble-free, wrinkleless coverings on surfaces of complex curvature without significant stretching. Davis applies these appliqués by:                (a) analyzing and mapping the Gaussian curvature of the surface to be covered to identify lines of constant Gaussian curvature;        (b) identifying geodesic lines on the surface, such that the lines of constant Gaussian curvature and the geodesics form a mapping grid on the surface;        (c) analyzing the sketchiness needed to blend between appliqués of adjacent areas of different Gaussian curvature;        (d) producing appliqués for each Gaussian curvature using a family of molds;        (e) identifying on the surface the grid made up of the lines of constant Gaussian curvature and intersecting geodesics; and        (f) applying appliqués of a particular Gaussian curvature along the matching line of constant Gaussian curvature on the surface to produce a complete, bubble-free, wrinkleless covering on the surface comparable to a conventional painted coating and while minimizing stretching of any appliqué to complete the coating.Identifying the grid can include physically marking the lines, displaying them with an optical template, or simply defining them in a three-dimensional digital data model for the surface.        
The Davis method recognizes that surfaces having the same Gaussian curvature can be mapped topologically. If you have a surface of Gaussian curvature 5 ft−2, for example, instead of making a “splash” mold of the surface to make appliqués, you mold appliqués to curvature 5 ft−2 on a master curvature 5 ft−2 mold, which, for example, might be a sphere. Appliqués from the master mold will fit bubble-free and wrinkleless on the actual surface.
Often surfaces must be protected against corrosion. Such protection commonly involves surface treatments or primers (that is, chromated primers or conversion coatings) that are relatively expensive because of the chemicals involved and the time associated with their application. These traditional coatings are relatively heavy, especially when coupled with other surface coatings that must be applied over the corrosion protection coating to provide color, gloss, enhanced surface durability, abrasion protection, a combination of these attributes, or other attributes. The chemicals used in conventional corrosion protection coatings often are hazardous materials.
Appliqués are of considerable interest today for commercial and military aerospace applications. Lockheed Martin and Minnesota Mining and Manufacturing Company have conducted flight tests on paintless aircraft technologies. These appliqués promise to save production costs, support requirements, and aircraft weight while providing significant environmental advantages. The appliqués are described in greater detail in the article: “PAINTLESS AIRCRAFT TECHNOLOGY”, AERO. ENG'G, November 1997, p. 17. Commercial airlines, like Western Pacific, use appliqués to convert their transports into flying billboards. Durable appliqués that can replace conventional military or commercial aviation paint systems are sought to reduce lifecycle costs, improve performance, and protect the underlying surfaces from corrosion.
The increasing interest in appliqués is described in the article: “REPLACING PAINT WITH TAPE FILMS”, AERO. ENG'G, March, 1998, pp. 39, 40. All the major military and commercial aircraft manufacturers are pursuing research and development programs to perfect appliqués. One important issue unresolved for appliqués is edge sealing. The seams between adjacent appliqués in some instances need to be treated for aerodynamic reasons.
A surface can be covered with appliqués to provide a vapor barrier and corrosion protection. The corrosion protection achievable with appliqués may be adequate to eliminate altogether the need for conventional surface corrosion protection treatments, thereby, saving weight and reducing environmental concerns. Alternatively, the combination of appliqués with environmentally friendly but relatively inferior, chromate-free conversion coatings may replace the environmentally sensitive, traditional corrosion protection techniques (that is, chromated conversion coatings and primers).
Corrosion on metal surfaces or around metal fasteners in resin composite structures produces oxidation that reduces the surface quality and that frequently can make the structural integrity suspect. Maintenance to correct corrosion or to ensure that it does not occur is costly because it is labor-intensive. A more reliable corrosion protection system would find widespread acceptance in commercial and military aerospace.
In addition to the corrosion protection that appliqués described above can provide, the vapor barrier can be beneficial independently on aerospace structure to limit the migration of water through a structure. For example, with composite honeycomb sandwich structure, a vapor barrier appliqué can slow or eliminate the migration of water through the laminated face sheets into the honeycomb core.
When making precision coatings that are important for aerodynamic drag and other considerations on modern commercial and military aircraft, spray painting is a relatively unreliable process because it is difficult to control the spray head and spraying conditions to obtain precisely the same coating from article to article. One variable in this spray process that often is overlooked is the natural variation from article to article in the vehicle to which the paint is applied. Such variation results from the accumulation of tolerances (that is, the accumulated variation that results from variations within allowable control limits for each pan in the assembly). Appliqués allow better control of the manufacture of the coating so that it will have the correct spectral properties by distributing pigments, additives, and thin films properly throughout the appliqué and, thereby, over the surface. The benefits of appliqués are further enhanced if the appliqués simultaneously provide corrosion protection. Difficulties in precisely manufacturing painted coatings to obtain the desired properties can be overcome without the cost of either scrapping an entire article because the coating is imperfect and inadequate or forcing costly stripping and reapplication of the coating.
Using appliqués allows small area repair of the precision coatings on aerospace surfaces by simply cutting away the damaged area and reinserting a suitable, fresh appliqué patch. With paint, the spray transition between the stripped area and the original coating in such a repair is troublesome. For example, an entire panel usually needs to be re-coated with paint to fix a small area defect. Operations like paint spraying, surface preparation, masking or otherwise isolating the repair area, and the like, slow the repainting process.